On NOMMU ARM, the __addr_ok() and __range_ok() macros do not evaluate
their arguments, which may lead to harmless build warnings in some
code where the variables are not used otherwise. Adding a cast to void
gets rid of the warning and does not make any semantic changes.

Without this patch, building at91x40_defconfig results in:

fs/read_write.c: In function 'rw_copy_check_uvector':
fs/read_write.c:684:9: warning: unused variable 'buf' [-Wunused-variable]
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Acked-by: NGreg Ungerer <gerg@uclinux.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>